[Feature]: Add `.editorconfig` + style analyzers

[samatstariongroup]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of the EcoreNetto
• I have searched open and closed issues to ensure it has not already been reported

Description

• Summary: The documented style rules (no regions, this. prefix, usings inside namespace, no underscore prefix) are only enforced by a ReSharper .DotSettings file; non-ReSharper contributors and CI don't enforce them.
• Affected files: repo root (new .editorconfig); optionally add StyleCop.Analyzers/Roslynator via Directory.Build.props.
• Proposed solution: Add an .editorconfig encoding the CONTRIBUTING rules and wire an analyzer package so violations show as warnings (optionally TreatWarningsAsErrors in CI).
• Acceptance criteria:
  • .editorconfig encodes the documented conventions.
  • Analyzers run in build/CI and flag violations.